It is known to use alkyd resins having high acid numbers for the production of varnish binding agents, which have good water solubility in the form of their amine salts. The disadvantages of systems of this kind, apart from their unsatisfactory waterproofness, are to be seen, in the majority of cases, in their considerable content of volatile amines and in some cases of toxic auxiliary solvents. When the varnish layer is baked, these substances must be trapped by means of suitable absorption systems.
It is further known to modify the varnish binding materials by the incorporation of hydrophilic chains, particularly polyethylene glycol chains, in such a manner that self-emulsifying alkyd resins or melamine resins result. Furthermore, even without incorporation in the resin molecule, it is possible to use polyglycol ether derivatives, such as the adducts of ethylene oxide to fatty alcohols, as low molecular weight, non-ionic emulsifiers. In both cases, however, the hydrophilic radical remains active in the baked varnish layer and, due to its hydrophilic nature, gives rise to a susceptibility to penetration by water. In addition, a marked reduction in film hardness also often occurs.